A diacetyl rest is done towards the end of fermentation to help speed up the tired yeast metabolism so that it can reabsorb diacetyl.
Traditionally this is done in Lagers where diacetyl is a big flavor flaw. Diacetyl will build up to a higher level in a cold fermenting beer
because colder liquids can absorb more and because there is less CO2 scrubbing out the diacetyl during subdued fermentations. The key
is to start the fermentation cold and then let the temp rise towards the end, in the case of an ale, raise the temp of a fermenting lager to
60